Grammar-forced speculative drafts: GBNF grammar as a third draft source, guaranteed-accepted forced spans, lossless + opt-in (#48, #70)

New byte-level GBNF-subset engine (c/grammar.h: parser + set-of-stacks PDA
walker) wired into spec_decode as a third draft source ("metodo F"), tried
before MTP/n-gram. Wherever the grammar admits exactly one legal byte, the
forced span is tokenized and injected as drafts; the existing batch-union
verification confirms them, so a wrong or out-of-sync grammar can never
change the output. Lazy arming skips preambles; adaptive guard (same
pattern as MTP) disables the source below 50% acceptance; grammar-accepted
tokens no longer pollute the MTP acceptance counter.

GRAMMAR=file.gbnf enables it in run and serve modes (also with DRAFT=0 and
with the int4 MTP head from #8); GRAMMAR_DRAFT=n caps the span (default 24).

Measured on M3 Max / int8-MTP container, greedy, MTP=0 DRAFT=0, NDJSON
classification: 0.37 -> 0.50 tok/s (1.60 tok/forward, 81 fw per 130 tok),
100% acceptance (48/48), output byte-identical to baseline.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -27,6 +27,7 @@ The engine is a single C file (`c/glm.c`, ~2,400 lines) plus small headers. No B
 - **MLA attention** (q/kv-LoRA, interleaved partial RoPE) with **compressed KV-cache**: 576 floats/token instead of 32,768 (57× smaller — GLM-5.2 has 64 heads and no GQA).
 - **DeepSeek-V3-style sigmoid router** (noaux_tc, routed_scaling_factor), shared expert, first-3-dense layers.
 - **Native MTP speculative decoding** — GLM-5.2's own multi-token-prediction head (layer 78) drafts tokens that the main model verifies in one batched forward. **The head must be int8** (the converter does this by default): at int4 draft acceptance collapses to 0–4% and speculation never engages; at int8 it's 39–59% acceptance, **2.2–2.8 tokens/forward** (community-measured, [#8](https://github.com/JustVugg/colibri/issues/8)). Lossless — *and stays lossless under sampling* via rejection sampling. Honest caveat from the same measurement: on a **cold** cache each verified draft routes to extra experts (~660 → ~1100 expert-loads/token), so speculation can be a net *time* loss until the cache/pin warms up — the adaptive guard and `DRAFT=0` are there for that.
+- **Grammar-forced speculative drafts** (`GRAMMAR=file.gbnf`, [#48](https://github.com/JustVugg/colibri/issues/48)) — on constrained-output workloads (JSON/NDJSON, function calling, structured extraction) the grammar itself is a third draft source: wherever it admits exactly **one** legal byte (braces, quotes, key names, enum bodies), that forced span is tokenized and injected as pre-accepted drafts with ~1.0 acceptance — no draft head, no lookup table, and it engages even with the int4 MTP head from [#8](https://github.com/JustVugg/colibri/issues/8). It never constrains sampling: forced spans are verified in the same batch-union forward as any draft, so a wrong or out-of-sync grammar cannot change the output — worst case is rejected drafts, and an adaptive guard turns the source off below 50% acceptance. Byte-level GBNF subset (literals, char classes, `| ( ) ? * +`, comments); `GRAMMAR_DRAFT=n` caps the forced span per forward (default 24). Composes with `DRAFT`/MTP, which fill the free-text gaps between forced spans.
 - **True sampling** — temperature + nucleus, defaults tuned for int4 reality (0.7 / 0.90; the official 1.0 / 0.95 samples quantization noise from the tail).
 - **Integer-dot kernels** (Q8_0-style int8 activations, AVX2 `maddubs`): int8 matmuls 1.4–2.5× faster (119 GFLOP/s measured), int4 1.8× in batch — routing decided per shape by measurement (int4 single-row stays f32: it measured slower).
 - **MLA weight absorption** (DeepSeek trick) for decode: no per-token k/v reconstruction — the query absorbs `kv_b`, context is projected after attention. Validated exact: TF 32/32 and generation 20/20 with absorption forced everywhere.
@@ -312,7 +313,7 @@ works against the colibrì OpenAI-compatible server (in review, #21) or any othe
 compatible endpoint. Nothing leaves the endpoint you configure. The terminal
 `coli chat` remains the first-class interface.
 
-Useful knobs (env or flags): `--temp T` token sampling temperature (default 0.7 + nucleus 0.90 — tuned for int4; 0 = greedy), `--topp 0.7` adaptive expert top-p (30–40% less disk), `--ngen N` max tokens per answer (`:piu` in chat continues a truncated one), `--repin N` adapt RAM/VRAM hot experts every N emitted tokens, `AUTOPIN=0` disable the learning cache's auto-pin, `THINK=1` enable GLM-5.2's reasoning block, `DRAFT=n` MTP draft depth, `TF=1` teacher-forcing validation, `PILOT=1` router-lookahead disk prefetch (experimental — see below), `CAP_RAISE=0` don't auto-grow the expert cache.
+Useful knobs (env or flags): `--temp T` token sampling temperature (default 0.7 + nucleus 0.90 — tuned for int4; 0 = greedy), `--topp 0.7` adaptive expert top-p (30–40% less disk), `--ngen N` max tokens per answer (`:piu` in chat continues a truncated one), `--repin N` adapt RAM/VRAM hot experts every N emitted tokens, `AUTOPIN=0` disable the learning cache's auto-pin, `THINK=1` enable GLM-5.2's reasoning block, `DRAFT=n` MTP draft depth, `GRAMMAR=g.gbnf` grammar-forced drafts for constrained JSON/NDJSON output (`GRAMMAR_DRAFT=n` caps the forced span), `TF=1` teacher-forcing validation, `PILOT=1` router-lookahead disk prefetch (experimental — see below), `CAP_RAISE=0` don't auto-grow the expert cache.
 
 **The expert cache auto-sizes to your RAM** (since 2026-07-10): the engine now *raises* the LRU cap to fill your `--ram` budget instead of only lowering it. Before this fix a 128 GB machine ran with the same 8-experts/layer cache as a 16 GB one (issue #12) — **if you benchmarked colibrì before this date, rerun: your numbers were capped.**